1. Field of the Invention
The present invention relates to a data modem for a detecting and cancelling line deterioration factor and particularly to a line signal deterioration removing system to be employed for data modem which effectively removes phase jitter factor.
2. Description of the Related Art
Data transmission utilizing a telephone line having a transmission band in the voice frequency range is recently spreading. In such a data transmission system, a so-called modem usually providing a modulator in the sending side and a demodulator in the receiving side is provided and various line deterioration factors generated in the line between the modems are removed by such modems provided in the sending side and receiving side.
Particularly, since the recent modulation system has improved the data transmission rate through introduction of the multiphase modulation system, it is desirable to equalize, with high accuracy, various kinds of signal deteriorations in relation to the phases of the signals to be transmitted. Such signal deteriorations include intersymbol interferences effectuated on the line, frequency offset generated due to a difference of carrier frequencies for modulation and demodulation, and phase jitter generated due to a fluctuation of power source voltage.
When the telephone line in the frequency range of 0.3.about.3.4 kHz is used, it is known that following typical deterioration factors must be considered.
(1) Intersymbol interference PA1 (2) Phase error and frequency offset PA1 (3) Phase jitter
For the equalization of characteristics which are considered as the deterioration factors, signal deterioration has been compensated in the receiving side with the structure shown in FIG. 11.
FIG. 11 is a structural diagram of an algorithm within the modem of receiving side of the prior art. Such structure is executed by programs using digital signal processing LSIs and microprocessors.
In the same figure, reference numeral 10 designates an automatic equalizer or (AEQ) 10 which is for example formed by a transversal filter. To an automatic equalizer 10, the base band signal after the demodulation or the pass band signal before demodulation which is received from the transmission line is input as the coordinate data (vector data) of signal points on the complex plane. The equalized output obtained by removing the intersymbol interference element which is the first deterioration factor 1 from such input signal is thus output.
In the successive stage of equalizer 10, a carrier phase controller 11 comprising a carrier automatic phase control circuit (or CAPC) 12 and a hard (or non-probabilistic) decision circuit 26 is provided. CAPC 12 foresees frequency offset and phase error included in the equalized output of equalizer (or automatic equalizer) 10 with the integral operation of quadratic form.
The output of CAPC 12 is multiplied, in vector form, with the equalized output of equalizer 10 and with the feedback signal to the equalizer 10 at the position where the conjugate vector * is indicated by the symbol at .circle. which denotes a multiplying point. The multiplication (second occurrence only) of the CAPC 12 output to the output of automatic equalizer 10 means stops phase rotation, i.e. demodulation and multiplicaiton, while the multiplication of the CAPC 12 output and the by feedback signal to the automatic equalizer 10 causes addition of phase rotation, i.e. modulation. The principle of removing carrier frequency offset with such structure is based on the principle that the phase of output of the automatic equalizer 10 rotates at a constant angular velocity due to the influence of carrier frequency offset. This rotation can be ceased by multiplying an output of CAPC 12 with the output of the equalizer 10.
As an input to the carrier automatic phase control circuit 12, a hard non-probabilistic decision error is given as a phase difference between the non-probabilistic decision point and receiving point by a non-probabilistic decision circuit 26. CAPC 12 computes frequency offset and phase error element included in the output of equalizer 10 based on such hard decision error. The computation result is multiplied with the equalizer 10 output. This structure is known as a non-probabilistic decision error feedback system because the non-probabilistic decision error is obtained from the output of the non-probabilistic decision circuit 26.
Following the carrier phase controller, a phase jitter remover 13 including a prediction filter circuit (or prediction filter ) 14 is provided.
The prediction filter circuit 14 predicts a phase jitter element by using prediction filter 14. An output of prediction filter 14 is multiplied to an output value of the carrier phase controller 11 at the part of multiplication code in order to remove the phase jitter element from the output of the carrier phase controller 11.
Line deterioration factors such as intersymbol interference, frequency offset, phase error and phase jitter are error-corrected and the revmoed receiving point information is finally applied to a soft (probabilistic) decision circuit or non-probabilistic decision circuit 16. The probabilistic decision circuit 16 decides, based on the input value, the most likely signal point which the receiving signal takes by the well known viterbi decoding method.
Moreover, in the case of non-probabilistic decision, like the preceding non-probabilistic decision, the signal points assigned corresponding to the regions assigned to each complex space are decided using tables corresponding to the values of respective regions.
Decision error given as an error between the decision point of the probabilistic decision circuit or non-probabilistic decision circuit 16 is fed back to the prediction filter circuit 14 as a normalized output for optimization control of the prediction filter circuit 14. Moreover, this error is also fed back to the automatic equalizer 10 through two multipliers.
In such a prior art system, intersymbol interference which is a signal deterioration factor of a transmission line is removed by the automatic equalizer 10, while frequency offset and phase error are removed by the carrier phase controller 11 and phae jitter is removed by phase jitter remover 13.
However, when a transmission rate of the transmission line to transmit data with the phase modulated signal becomes higher, data of respective signals are different even when the phases are very approximated, and therefore very accurate demodulation of signal phase is required.
The conventional modem does not require highly accurate demodulation and compensation for phase because the transmission rate is low, but when it is applied to demodulation of relatively high transmission rates, the signal deterioration factors cannot be removed perfectly, resulting in a problem that phase error becomes numerous.
At relatively high transmission rates, attempts have been made to eliminate the necessity of raising accuracy of signal phase during the demodulation by automatically compensating for an error with convolutional encoding, i.e., maximum likelihood encoding method such as viterbi encoding or multi-dimensional trellis encoding, etc. However, this method cannot accurately realize the phase compensation of a receiving signal and is not always recommended as a method of compensation.
In addition, the conventional structure explained above is preferable in situations in which high trackability for distortion of phase error can be obtained by using a non-probabilistic decision circuit 26 which can suppress delay within the loop to zero for the control of CAPC 12 by feedback of non-probabilistic decision error. However, when the higher trackability is obtained, a problem arises in that noise bandwidth of CAPC 12 becomes wider and S/N characteristic is deteriorated.
Therefore, it is desirable that the trackability be lowered and S/N characteristic be improved. However, in this case, undesirable large share is applied on the prediction filter 14 in the successive stage.
Accordingly, the optimization of S/N characteristic has been attempted by searching the intermediate point.
Moreover, the inventors of the present invention have found that even when various coefficients are optimized with such structure, a phase includes an error and highly accurate compensation is impossible.